Until recently seaweeds were described as simple plants that grew in the sea, but scientists now consider that the structure and chemistry of seaweeds is so distinctive that they warrant their own classification. They are simple in structure, usually consisting of a holdfast that anchors them to a surface, and a blade which may be divided into fronds. Some of the larger seaweeds have a flexible stalk or stipe connecting the blade to the holdfast. Unlike plants, seaweeds do not have roots or internal tissues to conduct water. They absorb minerals and gases directly from sea water through the surface of their blades.

Seaweed groups

Seaweeds can be grouped into three types, based on colour – green, red or brown. They all contain the light-absorbing pigment chlorophyll, which is necessary for photosynthesis. Brown and red seaweeds have additional pigments that enable them to photosynthesise at depths where little light penetrates. These extra pigments mask the green colour of chlorophyll. Brown seaweeds can be yellow-brown to dark olive. Red seaweeds have the greatest range of tone – pink to purple, red, and brown to nearly black.

Life cycles

Many seaweeds only live, or only grow, for a single season; others, especially the large kelps, grow year-round and may live for many years. Seaweeds have complex life cycles involving both sexual and asexual stages. Their appearance may change markedly between these stages. Red seaweeds have the most complicated life cycle. One edible species, karengo (Porphyra species), includes a phase that bores into the surface of shellfish and rocks.

Distribution

New Zealand has 850 native seaweeds, a third of which are endemic – they are not found anywhere else. Seaweed is particularly plentiful in three areas: the warm northern waters around the Kermadec and Three Kings Islands, the Cook Strait–Kaikōura coast region in central New Zealand, and the south, in an area encompassing Fiordland, Stewart Island and the Otago coast. Distribution varies according to species. Some, like the red seaweed Gelidium longipes, only grow in a few places. Others, such as giant kelp (Macrocystis pyrifera), are widespread in cool southern hemisphere waters.

Role in the marine ecosystem

Seaweeds play a major role in marine ecosystems. As the first organism in marine food chains, they provide nutrients and energy for animals – either directly when fronds are eaten, or indirectly when decomposing parts break down into fine particles and are taken up by filter-feeding animals. Beds of seaweed provide shelter and habitat for scores of coastal animals for all or part of their lives. They are important nurseries for many commercial species such as the rock lobster, pāua (abalone) and green-lipped mussel.

Types of seaweed

Green seaweeds

Green seaweeds are usually found in the intertidal zone (between the high and low water marks) and in shallow water where there is plenty of sunlight. About 140 species have been recorded around the coast. One of the easiest to recognise is sea lettuce (Ulva lactuca), which forms bright green sheets up to 30 centimetres in diameter. As its common name suggests, it is edible, although prolific growth often indicates sewage pollution. Sea lettuce can become a problem when large quantities are washed ashore and begin to rot, giving off an offensive sulphurous smell. Gut weed (Enteromorpha intestinalis), a tubular green seaweed, also favours high-nutrient sites. Another common green seaweed is sea rimu (Caulerpa brownii), also edible, and looking very much like the foliage of the large tree rimu.

Brown seaweeds

These medium to giant-sized seaweeds typically grow at depths below the greens and above the reds. Neptune’s necklace (Hormosira banksii) is well known to most people who have visited the rocky shore. Its branching chains of water-filled bladders help it withstand periods of exposure when the tide goes out. Many seaweeds produce mucilage or slime to protect against drying out. Of the brown group, Gummy weed (Splachnidium rugosum) takes mucilage production to the extreme – its swollen tubes ooze profuse quantities of sticky slime when touched. The largest brown seaweeds are known as kelps, and are prominent in the cooler coastal waters of southern New Zealand.

Red seaweeds

There are 550 species of red seaweed, making them the largest group. In the clear waters around the Kermadec Islands red seaweeds may be found at depths greater than 200 metres. In the nutrient-rich coastal waters of New Zealand’s main islands very few survive below 25 metres.

One of the best-known reds is the edible karengo (Porphyra species), which grows on rocks near high-tide level and resembles sheets of light purple cellophane. It is a close relative of the Japanese nori, used for sushi. Another familiar red is the fern-like agar weed (Pterocladia lucida) which has been harvested for agar production in New Zealand since 1943. The coralline seaweeds are a group of reds that deposit calcium carbonate in their cell walls, forming pink skeletons or paint-like crusts on coastal rocks. Scientists have discovered that some crust-forming seaweeds release chemicals that encourage pāua (abalone) larvae to settle and mature.

Kelp forests

New Zealand’s largest seaweeds are collectively called kelps and belong to the brown group. Because beds of kelp have such a complex structure and are able to modify their environments, Charles Darwin likened them to forests. Kelp forests may be over 20 metres tall, and support an understorey of smaller brown and red seaweeds as well as rich populations of both grazing and immobile animals.

Bladder kelp

The country’s largest kelp is Macrocystis pyrifera, known as bladder kelp in New Zealand, but giant kelp elsewhere. It can grow to 50 metres in length and 100 kilograms in mass. Bladder kelp forms large forests in the deep sheltered waters of southern New Zealand, and is easily identified by the gas-filled floats at the base of each frond. These help to hold the kelp upright, maximising the amount of sunlight the blades receive.

Common kelp

In northern waters common kelp (Ecklonia radiata) dominates marine forests. This seaweed is about 1 metre in length and has a bunch of fronds arising from a central supporting stem. It is the preferred food of the sea urchin, kina (Evechinus chloroticus), which sometimes mass together and munch through an entire forest. Following such large-scale habitat destruction, other seaweeds may dominate for a while, but in northern New Zealand, at least, common kelp grows rapidly and soon replaces itself.

Marine invaders

Seaweeds have been stowaways to New Zealand on the bottom of vessels or in ballast water for years. A few arrived with sealers and whalers in the late 18th and early 19th centuries, but did not pose a threat to native marine life. The situation changed when Undaria arrived. At two months of age it can release millions of spores into the sea. Germinating spores will colonise any firm surface – ropes, buoys, vessel hulls, floating plastic as well as rocky reefs – and grow rapidly, displacing native seaweeds.

Bull kelp

Bull kelp or rimurapa (Durvillaea species) is the most striking seaweed of the exposed coasts. Its tough, flexible fronds are secured to intertidal rocks by a solid disc-shaped holdfast capable of withstanding tremendous forces when storm waves crash onto reefs and cliffs. The seaweed grows to 10 metres in length and can live for 10 years. Bull kelp forests are highly productive systems, contributing vast quantities of organic matter and nutrients to coastal food chains.

Four species of bull kelp are found around New Zealand, and the most common, Durvillaea antarctica, also grows around the subantarctic islands and southern coasts of South America. In northern New Zealand it grows only on very exposed headlands and becomes more common in the cooler waters south of Cook Strait. Fronds of Durvillaea antarctica have an internal honeycomb-like tissue, full of air, which keeps the blade buoyant. The form of the frond differs according to conditions: the more exposed the site, the more divided the fronds.

Asian kelp

Asian kelp (Undaria pinnatifida) arrived in New Zealand waters in the 1980s and quickly made itself at home in sheltered harbours. Although it is farmed and eaten in Japan, where it is known as wakame, this fast-growing, 2-metre-tall kelp is unwelcome in New Zealand as it can change the structure and composition of native marine communities.